This invention relates to a method of forming a coating on a work surface of a machine component. The coating is of a metal material that is softer than the work surface of the machine component, and more particularly, to a method of forming a coating on a surface of a swash plate for use in a swash plate compressor to improve sliding contact between the surface of the swash plate and shoes.
Within a swash plate compressor, lubrication between sliding contact members that form an internal mechanism is generally provided by forming a mist of a lubricating oil in the compressor by using a gas (e.g. a refrigerant gas such as chlorofluorocarbon gas) flowing in the compressor, in accordance with operation of the compressor, and delivering the oil in the form of mist to the respective sliding contact members. However, at a restart of the compressor, for instance, after it has been left in an inoperative state for a long time, lubricating oil attached to the sliding contact members has often been washed away by the refrigerant gas. For this reason, immediately after the restart of the compressor, the sliding contact members, which need lubrication, are insufficiently oiled, in spite of operation of the compressor, over a time period (approximately one minute) until a sufficient mist of oil is formed by refrigerant gas that returns to the compressor. Therefore, conventionally, there have been proposed techniques of coating the surfaces of the sliding contact members to ensure minimum lubrication between the sliding contact members even when they are insufficiently oiled during the above-mentioned time period.
Conventionally, various techniques have been proposed, even in a limited field of art, in which a coating is formed on surfaces of a swash plate, more specifically, surfaces of a swash plate in sliding contact with shoes. Coating techniques not only disclosed in patent documents but also employed on products (swash plate) include electrolytic and electroless plating techniques using tin or the like and a spray coating technique using a copper-based alloy, an aluminum-based alloy, or the like.
In the electrolytic or electroless plating technique using tin, however, an extremely thin coating having a thickness of several micrometers (e.g. 1 to 10 xcexcm) can be formed without much difficulty, whereas a relatively thick coating having a thickness of several tens of micrometers (e.g. 20 to 70 xcexcm) cannot always be formed with ease. Further, in this plating technique, the electrochemical relationship between the base material and the metal to be deposited matters and can present problems. Thus, the technique cannot always be employed.
On the other hand, a spray coating technique, in which metal material in powder form is melted and sprayed together with flame onto a work surface does not encounter significant difficulties with regard to coating thickness or electrochemical relationship. However, the technique suffers from the following intrinsic problems in operations: In many cases, it is required to roughen the surface of a work surface of a machine component by shot blasting or the like before providing spray coating. Time and labor required for the pretreatment and a cost for hard particles (auxiliary material) used in the surface roughening treatment are factors increasing the time and cost for the spray coating. Further, considerable noise is generated during the surface roughening treatment, which degrades the working environment. Moreover, before providing spray coating, it is required to mask portions that do not need spraying, which further increases the time and cost. As described above, in spite of its high general versatility as a technique for forming a metal coating on a metal work surface, the spray coating technique has many problems to be solved with respect to the working environment on the shop floor as well as the labor, time and cost.
It is an object of the present invention to provide a method of forming a coating that is versatile, improves the working environment, and reduces the required labor, time and cost. Further, it is another object to provide a method of forming a coating that facilitates control of thickness of the coating.
The gist of the present invention consists in a method of forming a coating on a work surface of a machine component including a preparatory step of preparing a feed form made of a metal material softer than at least a work surface of the machine component, a pressure-welding step of pressure-welding an end face of the feed form to the work surface of the machine component while causing relative rotation between the machine component and the feed form, to thereby join the feed form to the machine component, and a cutting step of cutting the feed form in a direction substantially orthogonal to an axis of the relative rotation at a position near a joining position at which the machine component and the feed form are joined to each other, to thereby add a soft metal layer having a predetermined thickness to the work surface of the machine component.
According to this method, a soft metal layer having a predetermined thickness is formed on the work surface of the machine component by joining the machine component and the feed form to each other by friction welding, and subsequently cutting the feed form. Therefore, by selecting a cutting position for cutting the feed form as desired, it is possible to control the thickness of a metal coating on lit the work surface with ease. Further, it is possible to form a metal material coating having a desired thickness on the work surface of the machine component with ease at low cost.
Preferably, the method further comprises a surface-finishing step of carrying out surface finishing of the soft metal layer added to the work surface of the machine component. By providing the surface-finishing step after the cutting step, it is possible not only to improve the smoothness of the surface of the soft metal layer, but also to widen a range of selection in determining the cutting position for cutting the feed form, thereby facilitating the operation for cutting the feed form.
Preferably, the metal material forming the feed form has a lower melting point than the material forming the machine component. Also preferably, the material forming the machine component is a ferrous material while the metal material forming the feed form is an aluminum-based material or a copper-based material.
For example, the machine component is a swash plate for use in a swash plate compressor. The method of the present invention is an extremely suitable method of forming a coating on a surface of the swash plate. Preferably, the work surface of the swash plate machine component is a surface that makes sliding contact with a shoe.
The method may further include forming a solid lubricant coating containing a solid lubricant, such as molybdenum sulfide, on a surface of the soft metal layer added to the work surface of the machine component.